A cam driving chain transmission device includes a timing chain (hereinafter simply referred to as the “chain”) trained around a driving sprocket mounted on a crankshaft and a driven sprocket mounted on a camshaft. Such a chain transmission device further includes a chain guide pivotally provided on one side of the slack side of the chain to extend in the moving direction of the chain, and a chain tensioner for applying an adjusting force to the remote end portion of the chain guide remote from the center of pivoting motion of the chain guide, thereby tensioning the timing chain, and thus preventing loosening and flapping of the chain.
Such a chain transmission device may further include a fixed chain guide on the tension side of the chain to guide the movement of the chain to more effectively prevent flapping of the chain.
Known chain guides for adjusting the tension of the chain or for guiding the movement of the chain includes those of the type which guide the chain whiling being in sliding surface contact with the chain. However, such chain guides offer a large resistance to the movement of the chain, and thus increases a loss in torque transmission.
In order to avoid this problem, International Application Publication 2010/090139 proposes a chain guide including a guide base made of a synthetic resin and elongated in the direction of movement of the chain, a plurality of roller shafts having both ends thereof supported on the guide base so as to be arranged along a curved line, and rollers comprising roller bearings rotatably supported by the respective roller shafts, whereby the chain is movably guided by the rollers.
With this chain guide, since the plurality of rollers guide the chain while in rolling contact with the chain, the resistance to movement of the chain is small and thus a loss in torque transmission is small.
In the chain guide of International Application Publication 2010/090139, the chain is stretched in a straight line at its portions between the adjacent rollers, and is bent at its portions coming into contact with the respective rollers. Since the chain is stretched in a straight line every time after being bent, the chain tends to flap, thus producing noise and vibrations. It is desired to reduce such noise and vibrations.
An object of the present invention is to provide a chain guide which is capable of minimizing noise and vibrations of the timing chain, while minimizing a loss in torque transmission.
In order to achieve this object, the present invention provides a chain guide comprising a curved guide base extending along a direction in which a torque transmitting chain moves. The guide base includes, on a surface of the guide base opposed to the chain, a rolling guide portion comprising a plurality of rollers which are rotatable and configured to guide the chain while being kept in rolling contact with the chain, and at least one sliding guide portion comprising a curved surface.
The present invention also provides a chain transmission device comprising a chain trained around a driving sprocket and a driven sprocket, a chain guide as described herein (i.e., of the present invention) pivotally provided on one side of the slack side of the chain, and configured to guide a movement of the chain, and a chain tensioner configured to apply an adjusting force to an end portion of the chain guide spaced apart from a center of pivoting motion of the chain guide, thereby pressing said end portion against the chain.
In order to more effectively reduce flapping of the chain, this chain transmission device preferably further includes a fixed chain guide according to the present invention which is provide on one side of the tension side of the chain to guide the movement of the chain.
As described above, since the chain guide according to the present invention includes a rolling guide portion for guiding the chain while being in rolling contact with the chain, and a sliding guide portion comprising a curved surface, the chain is guided by the rolling guide portion while being in rolling contact with the rolling guide portion. This reduces a loss in torque transmission. Further, since the chain is guided by the sliding guide portion while kept in sliding contact with the sliding guide portion, and while maintaining a constant radius of curvature, the chain is less likely to flap and thus produces less vibrations and noise.
When the chain enters one end of the curved guide base, the chain, which has been moving in a straight line, is bent so as to have a radius of curvature. When the chain leaves the guide base from the other end thereof, the chain, which has been moving along a curved line with a radius of curvature, now begins to move in a straight line. Thus, the chain tends to vibrate both when entering the guide base from the one end thereof and when leaving the guide base from the other end thereof.
The tension of the timing chain fluctuates due to fluctuations in torque of the driving sprocket, which is mounted to the crankshaft, when the engine explodes, and due to fluctuations in torque of the driven sprocket, which is mounted to a camshaft, when the intake and exhaust valves are pushed in. Thus, the chain tends to vibrate more violently at its portions near the driving and driven sprockets. If, as in the conventional arrangement, the chain is in sliding contact with the guide base, this means that the chain is in surface contact with the guide, while if the former is in rolling contact with the latter, this means that the former is in line contact with the latter. Thus, if the chain and the guide base are in sliding contact with each other, the contact surface area is larger than if the chain and the guide base are in rolling contact with each other, so that the guide base can more stably guide the chain, and thus more effectively reduce vibrations.
For this reason, the at least one sliding guide portion is preferably provided at one of the end of the guide base at which the chain enters the guide base, and the end of the guide base at which the chain leaves the guide base, in order to effectively reduce vibrations of the chain with the sliding guide portion.
The guide base of a camshaft-driving chain transmission device is small in radius of curvature at its portion close to the crankshaft or to the camshaft, and a large load is applied from the chain to this portion, i.e., the portion which is small in radius of curvature. Also, a larger load is applied to the end portion of the guide base which is pressed by the chain tensioner than to its end portion about which the guide base pivots. By providing the rolling guide portion at such a portion of the guide base, i.e., the portion to which a large load is applied from the chain, it is possible to smoothly guide the chain while in rolling contact with the chain, and thus to effectively reduce a loss in torque transmission.
The at least one sliding guide portion of the chain guide according to the present invention may comprise a plurality of sliding guide portions disposed between the respective adjacent pairs of the rollers, which constitute the rolling guide portion. With this arrangement, the chain can be guided while being in sliding contact with the sliding guide portions and in rolling contact with the rolling guide portion, and while maintaining a constant radius of curvature. Therefore, it is possible to effectively reduce vibrations of the chain.
By providing the sliding guide portions at lower levels than the portions of the rolling guide portion which contact the chain, the chain is guided by the sliding guide portions, while being in sliding contact with the sliding guide portions, only when the chain vibrates. This reduces the resistance to movement of the chain, and thus effectively minimizes a loss in torque transmission.